Conventionally, as a method for measuring a surface temperature of an object such as a steel plate having a flat or curved shape, a contactless measuring method is well known. For an apparatus for use of such method for contactlessly measuring the surface temperature, there are surface-temperature measuring apparatuses, for example, an infrared thermography which detects infrared light emitted from the object.
The measuring accuracy for the temperature distribution of the surface of the object measured by the infrared thermography varies in response to the emissivity of the surface of the object. In detail, even under the condition that all of the object surface have the same temperature, if the emissivity of the object varies in the imaging area, there is a variation in the amount of the infrared light emitted from the surface of the object. That is, the amount of the infrared light detected by the infrared thermography is different in response to the emissivity, and therefore it is difficult to accurately measure the surface-temperature distribution.
For instance, a black body coating is applied on the surface of the object so as to make the emissivity constant (because the emissivity of the black body coating is nearly equal to 1), thereby reducing the influence of the variation in the emissivity (e.g., see PTL 1 to PTL 3).
Though the surface of the object is coated by the black body coating as the Patent Literatures, in the case that a material of the object has high coefficient of thermal expansion or that the heating/cooling process of the system is quick, the film of the black body coating may have cracks as shown in FIG. 8(a) caused by the thermal deformation (thermal expansion or thermal shrink) of the object. In such case, the thermal image that is produced based on the thermal image values of the object involves linear patterns caused by the cracks as shown in FIG. 8(b), because the emissivity on the crack section of the black body coating is lowered. As mentioned, the cracks unfortunately occurred in the film of the black body coating make the accuracy of generating the thermal image lowered.
Conventionally, for the case that the cracks occur in the film of the black body coating, there is a technique of smoothing the thermal image produced based on the values in the thermal image of the object (e.g., see PTL 4). In the Patent Literature, the smoothing for the thermal image is carried out by using averaging of the values.
In an alternative technique of smoothing, unexpected values caused by internal noise of a sensor are removed by comparing values around the unexpected value, and the removed values are interpolated by the average of the around values (e.g., see PTL 5).
In smoothing the values of the thermal image, it is preferable to eliminate fallings of the value caused by the cracks from the temperature profile data (L1) of the values of the thermal image, which is shown in FIG. 9 as a solid line, and to obtain a smoothed data (L2) showing substantially envelope of the values of the thermal image, which is shown in FIG. 9 as a dotted line. However, in the case where the smoothing is performed as PTL 4, the influence of the lowered values of the thermal image in the cracked areas cannot be eliminated, so that a smoothed data (L3) wherein the values of the thermal image have local falling, which is shown as a chain line in FIG. 9, is obtained. Therefore, it is difficult to accurately measure the surface-temperature distribution.
The technique of PTL 5 is related to removing the internal noise of the sensor, so that the unexpected values are produced in a different way. As a result, it is difficult to apply such technique to remove the influence on the thermal image caused by the cracks occurred in the film of the black body coating.